Oxides of nitrogen are one of the principal contaminants emitted by combustion processes, and various combustion processes are known to generate effluent gases having an unacceptably high NO.sub.x content. In every combustion process, the high temperatures at the burner thus results in the fixation of some oxides of nitrogen. These compounds are found in stack gases mainly as nitric oxide (NO) with lesser amounts of nitrogen dioxide (NO.sub.2) and only traces of other oxides, such as N.sub.2 O. Since nitric oxide (NO) continues to oxidize to nitrogen dioxide (NO.sub.2) in the air at ordinary temperatures, there is no way to predict with accuracy the amounts of each separately in vented gases at a given time. Thus, the total amount of nitric oxide (NO) plus nitrogen dioxide (NO.sub.2) and N.sub.2 O in a sample is determined and referred to as "oxides of nitrogen (NO.sub.x)".
Production of hydrogen chloride (HCl) is another undesirable side effect of combustion operations. Upon contacting water in the atmosphere, HCl forms hydrochloric acid, which contributes, as does NO.sub.x, to the acid rain problem.
The production of cement is a relatively complex process that involves mining and milling the raw materials, which are then fed directly into a kiln, or fed initially into a heat exchanger (typically a preheater or precalciner) which discharges the material into a kiln, and fired to produce "clinkers". The clinkers are subsequently milled and packaged for sale as cement. This prior art process is schematically illustrated in FIG. 1.
Cement kiln operations are among the combustion processes known to involve production of substantial quantities of undesirable NO.sub.x and HCl. NO.sub.x emission can be decreased by introducing a selective reducing agent, such as ammonia, into the combustion mixture.
With the technology currently available, reduction of HCl emissions requires the use of a wet scrubber. Wet scrubbers increase the complexity of a combustion process, and greatly increase the cost of combustion operations.
Various modifications of kiln operations have been attempted to reduce the level of emissions of pollutants, and to take advantage of the combustion process to incinerate undesirable waste as well. In particular, waste water treatment plant sludge (i.e., biological sludge) has been used as a feed material in cement kiln operations, which in turn eliminates the sludge.
For example, Yamane et al., (U.S. Pat. No. 5,217,624, issued Jun. 8, 1993) describes a process for introducing waste treatment sludge into a cement kiln, and introducing gases generated during processing of the sludge into the kiln as well in order to reduce NO.sub.x content of the exhaust gas. Yamane et al. specifically describes a process of mixing sludge with quicklime to produce slaked lime and sewage sludge. The mixture is dehydrated, releasing gases including ammonia. The remaining solids are introduced into a cement kiln, where the lime and inorganic substances become raw materials for cement, and the organic substances become fuel for cement burning. Dust and water vapor present in the gases produced during dehydration are removed by filtration and distillation, respectively, allowing introduction of the gas containing ammonia into the kiln. The gas containing ammonia helps reduce NO.sub.x emissions.
Lafser, Jr. et al. (U.S. Pat. No. 4,921,538, issued May 1, 1990) concerns a method for recycling and reusing contaminated earth and mineral matter in the manufacture of cement clinkers. The contaminated material is mixed with inorganic constituents in water to form a raw material slurry suitable for manufacture of Portland cement, and is then charged to a cement kiln.
Wuntz (U.S. Pat. No. 4,306,978, issued Dec. 22, 1981) concerns a method for lime stabilization of waste water sludge. The sludge is dewatered to produce a sludge cake having 10 to 60% solids and this is then mixed with calcium oxide to produce stabilized sludge pellets.
Steinbiss et al. (U.S. Pat. No. 4,640,681, issued Feb. 3, 1987) concerns a method and apparatus for removal of hazardous and waste materials of low heat content, such as refuse. Among the wastes considered are "household wastes, agricultural waste and industrial waste."
Enkegaard (U.S. Pat. No. 4,984,983, issued Jan. 15, 1991) concerns a method and apparatus for co-firing hazardous organic waste in solid pasty, greasy or sludge form by introducing it directly into the burning zone of an industrial rotary kiln and burning the waste in such zone simultaneously with cement kiln or lightweight aggregate. In particular, the waste is gasified prior to injecting the gas into the kiln. The gasification may be accomplished in a conventional gasifier such as is used for coal gasification.
Mozes et al. (U.S. Pat. No. 5,058,514, issued Oct. 27, 1991) describes a method for the simultaneous control of sulfur dioxide and NO.sub.x emissions from power plant flue gases. In the method described, an aqueous slurry of limestone and a nitrogenous progenitor such as urea is injected into the furnace at temperatures ranging between 900.degree. C. and 1350.degree. C.
Somewhat similar to the above, Amrhein et al. (U.S. Pat. No. 5,176,088, issued Jan. 5, 1993) discloses, among other things, the injection of ammonia into a furnace region having a temperature of about 1600.degree. to 2000.degree. F., to reduce NO.sub.x.
There is, therefore, a need in the art to reduce pollutant emissions produced during combustion processes, such as cement kiln operations. In particular, there is a need to reduce NO.sub.x emissions.
There is also a need in the art to reduce HCl emissions produced during combustion processes, particularly cement kiln operations.
There is a further need in the art to utilize and eliminate sewage sludge.
Another need in the art is to provide for an efficient and cost effective means for achieving both of these ends.
These and other needs in the art are addressed by the present invention, as described below.